So I received a phone call the other day from my sister. It seems that her and her husband are going to start working with bees. They went to a seminar and were told there that putting a fan in the bee hive / box will help with the production of honey. I guess this is because it will free up the bees to go gather nectar and such instead of trying to cool the hive with their wings. Anyway... They were guided to a website that sells these solar powered fans that are temperature controlled and mounted on some sort of enclosure that fits all the standard hive/box shapes. They sell for around $120 each. I was asked if I could come up with something that was cheaper.

I looked at the site they were looking at and the fan is nothing more that a computer fan that moves about 20 - 30 CFM. I'm thinking this is fairly straight forward. So I came up with a simple circuit with a thermistor, trim pot, and transistor. The question that I have is what kind of problems may be encountered with the circuit? The solar panel that I have is kind of small to run a fan, 6VDC open circuit and 120mA short circuit.

I did do a test build of the circuit just lighting an LED and it works decently. Though I did notice that when it is on the threshold of fully turning on the LED is flickering a little. Is there another solution to this that I am overlooking? I was trying to keep it simple. This circuit is about as simple as I can think of.

But your big problem is that you'll need more solar cell, and maybe less fan. There must be guides somewhere for how much power is needed for a given airflow. Start with an estimate of your target air flow and work backwards. Bee references might advise how many turnovers per hour you need, or something like that. Convert that to CFM and ultimately to watts.

Your current solar cell is probably far too small for that motor. I'm guessing you haven't tried it? It might not even turn when connected directly in full sun. Maybe 2 cells in series would have a chance.

But your big problem is that you'll need more solar cell, and maybe less fan. There must be guides somewhere for how much power is needed for a given airflow. Start with an estimate of your target air flow and work backwards. Bee references might advise how many turnovers per hour you need, or something like that. Convert that to CFM and ultimately to watts.

Your current solar cell is probably far too small for that motor. I'm guessing you haven't tried it? It might not even turn when connected directly in full sun. Maybe 2 cells in series would have a chance.

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I'll look at the link you have provided. As to my current solar cell, yes it is too small to power a fan. I was just using it kind of as a "proof of concept". I have already figured on getting something a little more powerful, perhaps 2 in series or parallel. Thank you for your input.

For your circuit, I think you'll want to use a MOSFET to act as a switch for the fan. On or off. The schematic you show would give a bit of proportional control by burning off some power in the controlling transistor. I think you'll want all the power you can get from the panel to drive the motor. You might even need a capacitor to hold enough juice to get the fan spinning when it turns on. Once spinning, it'll need less current than it needs to get started.

Oh, and what you're after is a LOT like the automatic cooling fans that folks are using in cars and boats to keep interiors cool while the sun is most intense. There are probably a lot of circuits out there by people that have rolled their own. No sense reinventing the wheel if you can find a good starting point.

This is just something my sister asked if I could build. I told her I could take care of the electronics side of things and she, i.e. her husband, could build the framework that it will go on.

The electronics side of it is fairly cheap excluding the solar panel. I found a solar cell/panel that will output about 22 VDC open circuit and is rated for 10 W for about $35.00. That isn't too bad of a price. When it is loaded it may actually be somewhere around 15 VDC, not too sure though.

I was thinking of an alternative to my first schematic. Use a LM35, or similar, and a comparator to drive a FET. Placing a trim pot on one input of the comparator will give the adjustment needed for the temp range. The issue that I have is that I have really never used MOSFET's before. So I am uncertain if a comparator will drive one. Would I just connect the output to the FET?

Short answer, yes. The MOSFET needs its gate to be fully driven to the saturated "on" state, eg. 10-15v for a normal MOSFET or 5v for a logic level one. So as long as you've got at least about 9v, you should be OK.

The other refinements to consider are a current limiting resistor on the gate control - in case the MOSFET fails or oscillates it won't short out the control. And you should also consider a pulldown resistor on the gate, to turn off the MOSFET if the control fails "open". Otherwise the gate could float.

But for a non-critical DIY system, a direct connection of the comparator to the gate will work fine. You will need a pull-up resistor to pull the comparator output high when it's output is up.